I'm not a theist, so I'm just arguing what-if kind of things, but my claim is that IF God exists then God is philosophically, not spatially, exterior to the universe. A more correct way to say it, I guess, would be to say that God would be prior to the universe. The point is that an omnipotent God can change the rules. He's bleedin' GOD, that's kind of the point of the whole God gig.
Not necessarily. The question regarded God destroying the UNIVERSE, not all the MATTER in the Universe. The Universe includes the spatial region itself. In my mind, when one talks of destroying the Universe, one is talking about, essentially, eliminating the space AND its contents (with space defined in the Schopenhauerian sense as the condition of the possibility of juxtaposition). With this context, and time understood to be a part of that spatial structure, if God eliminates that spatial structure, then there is no reason for the physical laws to have to be obeyed.
Of course, you get into weird places at this point. If God has eliminated the spatial structure of reality AND reality's contents, then the whole question of this happening at a particular point in time becomes bizarre and meaningless and just plain confusing.
My point is just that if we understand God as an omniscient critter who created everything and made up all the rules, He can CHANGE the rules, too. So it'd be impossible to predict what the results would look like if God decided to end the universe.